What i'd like to be able to do is to subtract background on the photo created by a motion detection camera.
Camera is statically mounted and the background expected unchanged most of the time, except when motion is detected.
'say it takes a picture of background only, then another object appears on the next picture,
then i'd like to programmatically cut only the area with that new object.

I understand that these backgrounds will not be binary equivalent since they are taken by a camera, sun moves,
wind blows - it will change slightly. So some deviation should be taken into account when detecting background similarity.
Is that too big of a problem to solve by Imagemagic ? Any leads ?

It just so happens I was just reading about this. Try to search "natural image matting". One top solution is called "Spectral Matting". There's a paper and Matlab source code, so this isn't an easy solution, it's for programmers.

I believe such operations are available in high-end video editing programs like After Effects.